Defense Secretary Caspar W. Weinberger, trying to prevent the House from derailing the military procurement bill, said yesterday that he would not request more money to put MX missiles in existing Minuteman silos.

He not only accepted Congress' cap of 50 MXs in Minuteman silos but promised not to request money in next year's budget for deploying the missile. Instead, the Defense Department will content itself with studying alternative basing modes for the MX until fiscal 1988, he said.

"We have no problem with the House-Senate conference decision on the MX that puts a limit of 50 MXs to be deployed in existing Minuteman silos," Weinberger wrote House Armed Services Committee Chairman Les Aspin (D-Wis.) yesterday. Aspin had threatened to hold up the fiscal 1986 military authorization bill if he could not obtain such a commitment from Weinberger.

Weinberger then took the unusual step of making a commitment for the coming fiscal year, telling Aspin that "the Defense Department plans for the fiscal 1987 program include no funding for any deployment of additional missiles over and above the 50 authorized by the bill as agreed by the conference.

"The fiscal 1987 request at this time is for the 21 missiles specifically approved for testing and spares and for our research program on basing modes, both in connection with the MX and the small intercontinental ballistic missile," Weinberger wrote.

The dustup between Aspin and Weinberger evolved after the defense secretary said he believed that the United States must deploy more than 50 MXs and that he would try to persuade Congress to meet that need.

Pentagon officials tried to avoid a political confrontation by stressing that Weinberger was talking about the 51st MX and future ones that might be deployed if a basing mode less vulnerable than existing Minuteman silos were found.

Weinberger and Aspin planned to meet on the issue yesterday but did not, and Weinberger sent his letter to Aspin later after several attempts to meet were unsuccessful.

Aspin has been assailed by some House members for giving in to the Senate too frequently in the conference to devise the military procurement compromise.

The Senate has passed the conference report setting forth the compromise measure, but the House has not voted on it.

In discussing the possibility of finding a more acceptable home for MXs beyond the first 50, Weinberger continued his conciliatory tone, writing Aspin:

"We will of course keep you and your colleagues fully informed of the progress of our basing mode studies so we can all reach an agreed approach for fiscal year 1988. When you and I meet, we can discuss, as you desire, any other aspects of missile basing."

It was not clear last night whether Weinberger's letter would be enough to head off a challenge to the bill by a group of Democrats protesting that Aspin conceded too much to Senate conferees.

"We were trying to agree on our position," Rep. Barney Frank (D-Mass.) said after meeting yesterday with nine House colleagues who have expressed unhappiness about the defense bill's size.

They expected to make their case at a caucus of all 253 House Democrats today, he said, but the meeting was canceled. Aspin requested the cancellation, according to congressional sources.

Besides objecting to the Senate MX cap at 50 rather than the House-approved 40, Frank and his allies contend that the authorized total of $302.5 billion is too high by about $10 billion. They also have complained that House-passed "reforms" in defense contracting were watered down in conference.